This present invention relates generally to spatial light modulators. More particularly, the invention relates to a method and structure for fabricating and/or operating a spatial light modulator with a high stiffness torsion spring hinge. Merely by way of example, the invention has been applied to initializing the micro-mirrors of a spatial light modulator using a resonant activation process. Additionally, the invention has been applied to a method of operating a spatial light modulator in a display application using a dynamic switching mode. The method and structure can be applied to spatial light modulators as well as other devices, for example, micro-electromechanical sensors, detectors, and displays.
Spatial light modulators (SLMs) have numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. Reflective SLMs are devices that modulate incident light in a spatial pattern to reflect an image corresponding to an electrical or optical input. The incident light may be modulated in phase, intensity, polarization, or deflection direction. A reflective SLM is typically comprised of an area or two-dimensional array of addressable picture elements (pixels) capable of reflecting incident light.
Stiction is a common problem encountered in contacting MEMS devices such as spatial light modulators. A micro-mirror light modulator is one example which in operation switches rapidly between two rotated position (on and off). The micro-mirror is typically supported by a torsion spring hinge and actuated by a bias voltage applied between the mirror and one of two electrodes (for a binary mirror). The electrode drive force is attractive and the micro-mirror is typically stopped by a landing pad or post. The force (torque) of the torsion spring will restore the mirror to its equilibrium (un-rotated) position when the bias voltage is removed or reduced.
An adhesion force occurs when the micro-mirror contacts the landing pad, know as the stiction force. The origin of the stiction force typically arises from capillary force, electrostatic force due to contact potential or dielectric, and Van Der Walls force. The relative contribution of these three components depends on the geometry of the contact as well as the materials of the contact and the environment the device is operating in. Thus stiction is a complex problem. When the stiction force equals and exceeds the restoring force of the torsion hinge, the micro-mirror will stick to the landing pad and causes failure of the device. When the stiction force varies in time (due to charging for example) it will affect the dynamic performance of the mirror, such as its switching speed.
A number of solutions have been applied to reduce the stiction of a MEMS device. A typical example is to apply a molecular layer on the interfaces to reduce the adhesion force between the contacting surfaces. Coating of some type of SAM molecules to convert the surface to be hydrophobic is one method that effectively reduces the capillary force between the contacting surfaces. In alternate methods of the prior art, a stiff landing spring is added as a landing pad, in addition to the torsion hinge of the mirror. An overdrive pulse is applied to actuate the landing spring (bending mode) which will then bounce the micro-mirror away from the contacting point (landing pad) and the torsion spring will restore the mirror to its non-rotated position. A combination of two or more anti-stiction solutions are often employed to improve the device performance as well as its reliability.
In principle, so long as the restoring force and the torsion spring force (Fhinge) is greater than the stiction force (Fstiction) then the device should function properly. To increase the stiffness of the torsion spring, one can increase its cross-section and reduce its length. This approach is particularly applicable for a torsion spring hinge made of materials with high Young's modulus and yield stress. A single crystalline silicon hinge is ideal for implementing this concept as its Young's modulus is more than twice that of aluminum and its yield stress, more than ten times that of aluminum.
Solving the stiction problem with high stiffness torsion spring might appear to be the simplest solution. In general it also improves performance (by increasing the resonant frequency) and the manufacturability of the device. However, there are undesirable consequences of a stiff hinge. First, it requires higher actuation voltage to pull down or switch the micro-mirror. Second, the high actuation voltage will lead to more acceleration of the micro-mirror during switching and hence more impact of the micro-mirror on the landing pad, which will accelerate the wear and increase the stiction.
Thus there is a need in the art for methods and systems to overcome these drawbacks and to allow a practical implementation for using stiff torsion springs to overcome stiction forces and improve long term reliability in a MEMS device.